Playstation 4 - Frame Rate Issue Problem

[TouchandFeel]

Do the frame-rates stay that low or do the start to climb back up and stabilize?

This could be a number of things causing this.
1) as mentioned it could be due to the recent population boom caused by the free login campaign as well as the release of the gold saucer. As far as I can tell, characters are some of the most taxing things on the system so large groups of them, such as those around player hubs will cause a noticeable dip in performance.
2) It could just be the occlusion/loading causing the frame-rate drops and you never really noticed it before or it has become compounded and therefore worse due to #1 above. When first entering into or the system loading an area, there will often be a noticeable dip in frame-rate as the system begins to cache and render the assets for that area. Often the frame-rate will stabilize as the assets and characters are cached and loaded in, but if you move away from the area and the system unloads the areas you will notice that dip when you move back since the system is then having to load them again. Ignoring player population, some areas are just more system intensive than others and will have cause lower frame-rates due to them being more graphically fancy-schmancy and eating up more resources or them not being optimized quite as well as other areas.
3) While the most unlikely, your system could be having hardware issues. Is it only this game that is having decreased performance? Is your console running hotter than it used to? Are there any other strange random graphical anomolies occurring? Any and all of which may point to something up with your system. Again, this is the most unlikely possibility but shouldn't be ruled out.

IMO it is most likely a combination of #1 and #2.
Even I, playing on PC, have noticed a more pronounced drop in frame-rate and area loading time recently, but it is always in areas that players gather and I have definitely noticed an increase in the number of players milling about in these areas.

Best of luck with getting this all figured out.